Selecting the power source for load using MOSFETs only?

Question

I'm looking for the circuit to automatically switch the load (< 0.5 A) between the main power source (USB) and back-up (battery) without a specialized IC and without the functionality of charging the battery.

I found two options:

• with 2 diodes
• diode from the main source and switchable P-MOSFET from the battery

I wonder if a similar solution can be implemented with MOSFETs only and came up with the following circuit:

Where VDC is the external power source, +BATT is voltage from battery, and VCC is the output voltage (automatically switched).

To my understanding (I'm not an EE) this should work as follows:

• if VDC is connected:
  • the top P-MOSFET connects the VDC to VCC since the lower N-MOSFET shortens the gate of top P-MOSFET to GND and opens it
  • other P-MOSFETs are closed.
  • VDC = VCC
• if VDC is not connected:
  • the VDC is pulled to GND, it opens the left bottom P-MOSFET, which pulls the top P-MOSFET to the VCC and closes it to prevent the back-flow from VCC to VDD
  • P-MOSFET connected to battery is open and connects the battery to load
  • VCC = +BATT

My questionas are:

• Is it going to work? In case if VDC is USB and +BATT is 3xAAA batteries? Or if +BATT is a LiIon battery?
• Is this solution any better in terms of saving energy than option with 2 diodes and/ one diode and mosfet? Are the voltage drops lower in such schematic?
• Are there any other problems in this schematics except the hustle to find P-mosfers which can be open from small batteries voltage?
• This circuit is going to work only if VDC > (+BATT + body voltage drop on right bottom P-MOSFET)?

Answer 1

In principle this circuit should work, however here are two suggestions:

• Remove the \$100k\Omega\$ resistor between VCC and the leftmost PFET. This resistor is slowing down the gate charging of the upper PFET during the VDC to BATTERY switchover. Meaning that, for a couple of miliseconds the battery might discharge into VDC if the latter \$VDC< V_{BAT} - V_{BD}\$.

• Add some capacitance at the node VCC in order to account for voltage dips during the switchover.

Update#1

The following simulation introduces a buffering capacitor \$C_1\$ which speeds up the gate capacitance charging of the upper PFET. Essentially the capacitor \$C_1\$ is charged when the VCC is active, and once the latter is disconnected, the capacitor discharges into the gate capacitance of the upper PFET.